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Grounding: should Vss = chassis?

Discussion in 'Electronic Design' started by Richard, Jul 29, 2003.

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  1. Richard

    Richard Guest

    I'm building a comm device that'll be housed in a metal chassis and
    rack-mounted. The question at-hand is whether the PCB ground should be
    tied to the chassis or left floating.

    Is there any benefit to tying the PCB ground to the chassis ground?
    Conversely, are there drawbacks? From my reading, I'm not sure that
    it'd reduce EMI emissions, and I'm concerned that it may expose Vss to
    external interference. There doesn't seem to be a safety issue, but are
    there certification mandates (e.g., UL)?

    FYI, the incoming line power will be appropriately grounded to the
    chassis for safety. And the metal shells of jacks (RJ-45) would be
    grounded to the chassis, not Vss.
     
  2. John Larkin

    John Larkin Guest

    I'd vote to bolt the PCB to the chassis in as many places as possible,
    which usually turns out to be all the mounting holes. The mounting
    holes on the PCB should be nailed right into the ground plane. That's
    what we do with all our high-speed (and even low-level analog) stuff.

    If you do anything else, you are explicitly hoping that the PCB ground
    plane will be at different RF potentials from the nearby chassis and
    the RJ-45 common mode RF level, and that doesn't make sense.

    John
     
  3. Genome

    Genome Guest

    There is no simple answer.

    You have to look at your application, buy the relevant standards, read them
    and apply according to your own interpretation.

    DNA
     
  4. Bob

    Bob Guest

    Don't let it float away 'till something breaks down. tie PCB Vss to chassis
    with ~10 Ohms or so.
    Try to remove "ground" from your vocabulary. Everything is differential.

    Are there any single-ended signals going in or out of the box? They usually
    require special attention if pickup or ESD are possible. Figure out where
    the "reference node" of each input or output is and treat it as a signal. Of
    course, if all your connectors are RJ-45 (transformer coupled), you probably
    don't have anything to worry about.
    That depends on to whom you are trying to sell it.
    "bonded" to the chassis is a better term.
     
  5. Richard

    Richard Guest

    Now, I understood that signals shouldn't be tied together at multiple
    points (IIRC, because of the differences in potential that can exist
    between the points) - is there an exception to this rule for linking
    ground planes?

    Perhaps the Dumb-Question-of-the-Day, but why is that a Bad Thing? Does
    the difference in RF potential have a negative effect on the PCB signals
    that wouldn't exist if they were coupled? I mean, there will always be
    a differential between the PCB and some part of its surroundings - what
    dictates where the coupling effort stops?
     
  6. John Larkin

    John Larkin Guest

    It's not a rule, it's a superstition.
    Well, if the PCB is flapping around at umpteen MHz relative to a metal
    enclosure that may be a fraction of an inch away, it will have signals
    induced into its traces, which may or might not matter for a
    particular situation. But there's certainly no advantage. The box is a
    nice farady cage protecting the PCB from external nasties (and from
    radiating as well) but that's lost if the PCB has a high impedance
    relative to the box it's in, and the PCB has wire connections to the
    outside world where RF can enter and leave, and nice ESD zaps can pump
    the PCB ground plane. The box stops being a friend and become the
    enemy.

    Single-point grounding is sort of an audio myth; at high frequencies,
    everything - including those nice star runs to the single-point thing
    - is an inductor.


    John
     
  7. Note that if this was an audio mixing desk, this would be suicide. In
    these cases, decent pro desks usually have *everything* isolated from
    chassis, with only one connectable lead to connect the electronics to
    the chassis.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  8. Not for audio.
    No way. With all due respect here, your mistaken.
    High frequencies are not the problem with audio. Its usually ground hum
    and dc offset switching clicks. With a sloppy ground connected
    everywhere, you are almost certainly *guaranteed* to have hum problems
    and offset clicks. My own power amp needs the ground lead from its
    capacitor centre point removed from the chassis to eliminate hum. Its a
    fact, I can generate the effect at will, so there is nothing
    superstitious about it at all. If the back plate cannons were grounded
    separately, along with the amp and meters etc, etc, there would be no
    practical way to lift the ground. Ground loops are a never ending
    nightmare in audio.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  9. Active8

    Active8 Guest

    i was (superstition) with a switcher i had with the output filter ground
    tied to the regulator chip's MOSFET ground with a trace. damn noise had
    the MOSFET stuck full on, i guess. running two wires for each ground to
    the bench supply's ground fixed it. weird, 20kHz switching. too low for
    ground bounce. must have been the circulating current. but such a short
    trace! i never did analyze that cause/effect.

    regards,
    mike
     
  10. R.Legg

    R.Legg Guest

    Wi'leenks? RF don' need no steenkeen wi'leenks!

    http://www.analog.com/ see AN214,AN345, AN346, AN347 and AN358

    http://www.lundahl.se/pdfs/papers/groundin.pdf

    Perhaps your local technical college or university library still has
    something by Ralph Morrison.

    RL
     
  11. John Larkin

    John Larkin Guest

    Sometimes they are, as when RF gets into low-level stuff, gets
    rectified by junctions, and becomes noise or DC offset. Lots of cheap
    telephones, for example, don't work right near radio/TV stations.
    Hum (and microvolt DC offsets in things like thermocouple amps) can be
    handled locally without enforcing a star grounding system. I make NMR
    gradient amps, 10 to 100-amp wideband current sources; they must have
    sub-PPM hum levels or I catch hell. Audio is gross compared to this
    stuff. I ground everything to the chassis to minimize RF and
    digital-analog crosscoupling, and handle all low-level stuff against
    the local ground. As long as you accept that the "ground" plane has AC
    and DC gradients, it's not hard to work around. These things are never
    "nightmares"; we design them and they work.

    John
     
  12. Ho humm...not in the context that I am obviously discussing.

    I was not suggesting that you *everything* has to be connected as a star
    ground. What I am saying is that connecting all bits and bobs to the
    chassis whenever you feel like it, simple don't work in many cases. If
    it does work, you were lucky.

    I have just given you a *real* example where if all connections had been
    connected to the chassis, I would be sitting here with bloody humming
    amp.
    So what. Pro audio usually has many pieces of equipment all connected
    together, all running from different power points. People have spent
    days trying to fix grounding problems in studios.

    I ground everything to the chassis to minimize RF and
    I have to disagree. I am very, very familiar with the practical design
    of pro audio gear and what actually works in practise. Its not
    debatable. Without being able to ground lift, you will have problems,
    unless your name is Jesus.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  13. But you were the one to suggest that for audio it was superstition, I
    was disagreeing with this view on that issue. My real life experience is
    quite the opposite. I bet your most pro audio companies will tell you
    right of the bat of the mess-up that had when they built their first
    products by earthing everything to chassis.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  14. Bob

    Bob Guest

    Consistently different potential is the right idea. If you don't make *any*
    connection between chasis and PCB Vss (or Vcc or whatever) then you have
    built a capacitor. The sum of the leakage currents and the (uncontrolled and
    maybe nonlinear) shunt resistance will determine the actaul potential
    difference. The better your isolation, the weirder things get.
    I don't think you really have anything to worry about then. Build the thing,
    test the thing, and decide for yourself how you want to connect the chassis.
    Just remeber to test it thoroughly. Hit it with everything that could ever
    happen in the customer's site (and then some). One of my favorite devices is
    our "cattle prod". It's a HV PS hooked up to a PVC pipe handle with a wire
    wisker sticking out the end. Inside the PVC is an RC (don't remember the
    details - we got it from a testing standards doc years ago and now it's
    potted inside the PVC ;-) circuit that simulates a human finger. Run the HV
    PS up to a few kV and poke the wisker at everything you can (connector pins
    and shells, front panel controls and indicators, chassis, EVERYTHING). It
    makes a satisfying poping sound and a nice little spark. Lot's of "well
    thought out" grounding schemes need re-work after the cattle prod is
    used.Signal integrity testing is a whole 'nother thing, but I'll stop now.

    Bob
     
  15. John Larkin

    John Larkin Guest

    OK, if you're interconnecting a bunch of store-bought gear that was
    designed by different people, you can expect ground loop problems, and
    you'll have to futz around until they go away. But if you're
    *designing* a precision instrument and control everything, hard
    chassis grounding can be managed nicely, and has a number of
    mechanical and ESD/RFI advantages. For an all-digital design,
    single-point grounding is not just silly, it's almost meaningless.

    The real point is to design it a way that will work, not to follow
    "rules" that propagate by hearsay.

    John
     
  16. Ben Bradley

    Ben Bradley Guest

    Even at 50 Hz down to DC, everything, including the chassis, is a
    resistor.
    Inadvertent RF pickup and rectification are easily controlled in
    audio with good RF filtering and shielding. Cheap products (the
    mention of which got snipped) generally don't have any
    filtering/shielding not required to make the product pass RF emissions
    tests.
    I've found AN003 and AN004 from this page to be very well written
    articles on where hum in audio systems comes from and how to fix it:

    http://www.jensen-transformers.com/apps_wp.html

    I see two basic problems: leakage current in power transformers,
    and sensitivity of input (and output!) connections to currents into
    the ground connection. Of course this applies within a case as well as
    when connecting different equipment. Even small currents through a
    chassis can cause a voltage drop that gets amplified by a low-level
    circuit. The solution is not neccesarily just blind "star grounding"
    but seeing where currents go, what is sensitive, and routing grounds
    where they (and the currents they carry) won't cause a problem.
     
  17. Active8

    Active8 Guest

    ok. add small currents/emi near high impedance inputs.

    mike
     
  18. Active8

    Active8 Guest

    Vss... was it about ground bounce, where a fast current transition in
    the high L bonding wire to a MOSFET source causes a voltage drop across
    it, thereby lowering the Vgs to the point where the thing cuts off?

    mike
     
  19. Which is usually the case in pro audio, or any audio for that matter.
    But you still have to be very carefull. Potentials on ground plans can
    still be such that noise gets in relativly huge amounts. Suppose one is
    designing a medical ultrasound imaging system:), and the effective
    input noise is << 1nV/rthz at Mhz BW. A Logic switch of 10ma in 10ns in
    5nh causing a 5mv spike would be completly hopeless. They is no real way
    you can elimitate this sort of noise without making sure the analogue is
    single pointed. Even 0.1nH of common impedance will cause severe bother.
    Obviously. 1 Volt noise immunity verses 1uV means that completely
    different approaches can be used.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
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